Method of preparing photographic emulsions by coagulating with gelatin graft polymers



METHOD OF PREPARING PHOTOGRAPHIC EMUL- SIGNS BY COAGULATING WITH GELATIN GRAFT PQLRS John W. Gates, Jr., John R. Dann, Bernard D. lllingsworth, and William G. Lovett, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application January 17, 1955,

' Serial No. 482,416

5 Claims. (Cl. 96114) This invention relates to the preparation of photographic emulsions having silver halide as the sensitive material in which the silver halide is prepared in dispersed form in a graft polymer of the. type which will be described herein. The polymer-silver halide dispersion is readily coagulated by adjustment of the pH. This invention also includes the products obtained thereby and the emulsions obtained by redispersing the coagulated silver halide grains in .a suitable vehicle.

The most common method of preparing silver halide dispersions is by reacting a water soluble salt such as silver nitrate and a water soluble halide such as potassium bromide in an aqueous solution of gelatin. There is formed thereby a dispersion of the silver halide in an aqueous solution of gelatin which solution also contains soluble salts resulting from the reaction of the halide and the silver nitrate. The emulsion maker has in the past been limited in the scope of his operations by the physical pnoper-ties peculiar to gelatin. These physical properties have necessitated the use of concentrations of silver halide and of gelatin in photographic emulsion formula which may not be the mostdesirable for all photographic products. For instance, emulsions having high concentrations of silver halide have been diflicult to prepare. Limitations have beenv imposed on the emulsion maker in the conditions of dilution during emulsion making since emulsions with a high water content are difficult to coat out and dry.

It has been considered desirable in the case of photographic emulsions particularly those to be coated on Water impermeable supports that the proportion of soluble salts present in the emulsion be diminished. It has been the practice heretofore Where gelatin has beenused as the peptizing agent for silver halide dispersions to remove the soluble salts therefrom by setting the gelled dispersion, noodling and washing the thus formed noodles with cold water. For this purpose a gelatin concentration of at least 4% is useful under practical conditions but this limitation has restricted the emulsion maker in the preparation of washed photosensitive emulsions by this method.

One of the objects of our invention is to prepare dispersions of silver halides from which the water soluble by-products may be readily removed. Another object of our invention is to provide a convenient andpractical method of preparing silver halide dispersions in which the dispersing colloid need be but a small percentageof the total compositions. A further object of our invention is to provide a method of washing silver halide dispersions in which substantially all of the liquid may be removed thus making for substantially complete removal of the water soluble impurities therein. Other objects of our invention will appear herein.

We have found that certain graft type polymers are useful as peptizers in the preparation of dispersions of silver halide and that the dispersions so prepared are coagulable within a specified pH range and will develop the silver halide grains when so precipitated. The method States Patent 0 Patented Apr. 2, 1957 of our invention avoids the limitations which have previously characterized the preparation of silver halide dispersions and is valuable in the manufacture of photographic silver halide emulsions of different speeds, cont-rasts and other photographic characteristics. In addition our invention shows a simple means of preparing emulsions of high silver concentration, this being especially valuable in the manufacture of photographic film having large quantities of silver per unit area.

The polymers which are useful as dispersing or peptizing agents for the silver halide in accordance with our invention are those which are prepared by a graft polymerization in an aqueous system comprising 60-100% of a hydrophilic material ofv which a saturated protein material either protein itself or a protein derivative constitutes at least 5% and 40-0% by weight of a hydrophobic material. The hydrophilic materials are completely soluble in water, other materials being designated hydrophobic. The monomers which are polymerized with the protein material are either of the vinylic or the diolefinic type as will be defined more specifically herein. The monomeric material employed may be either hydrophilic or hydrophobic and should constitute at least 5% of the total solid material being polymerized. If the monomer is a hydrophilic monomer it can amount to 595% of the weight of the mixture or if a hydrophobic monomer is employed it may amount to 15-40% of the weight of the mixture or both a hydrophilic and a hydrophobic monomer may be employed, the total of the monomers constituting at least 5% of the solid material which goes into the polymerization reaction. The polymerization is desirably effected in aqueous medium and the products thus obtained are compatible with photographic gelatins and degraded gelatin and thus are useful in the preparation of gelatin photographic emulsions. These materials are miscible with other types of materials which have been employed as vehicles in photographic emulsions such as polyvinyl alcohol, polyvinyl acetals, carboxylated polyvinyl acetals, hydrolyzed cellulose esters and the like.

The polymers which are prepared in our invention are actual chemical combinations of a protein component and one or more polymerizable monomers and appear to be formed by a type of graft polymerization between the growing polymer chain and the protein molecule. These polymers consist of at least 60% by weight of combined hydrophilic material which can be saturated protein material alone or saturated protein and a hydrophilic monomer. Some examples of hydrophilic monomers which are useful in this connection are acrylic acid, methacrylic acid, acrylamides, vinyl pyridine hydrochloride and the like.

The saturated protein component which is employed in preparing the polymeric materials can be an unsubstituted protein such as gelatin, hydrolyzed gelatin, glue or casein or soybean protein or the like or it can be an acylated protein such as succinyl, phthalyl or acetyl prepared by reacting upon a protein such as .glue, gelatin or casein with an organic acid anhydride or chloride. Types of proteins which may be employed are those resulting fuom oxidation with hydrogen peroxide as described in Lowe and Gates Patent No. 2,691,582. One advantage of our invention resides in the fact that the protein employed to peptize the peptizing agent need not be in a highly purified state or of a high degree of polymerization. Thus low grades of gelatin, glue or casein or the like can be employed in preparing peptizing agents in accordance with our invention.

The preparation of graft polymers suited for use in accordance with our invention employing vinylic or diolefinic monomers is described and claimed in applications Ser. Nos. 426,551 and 426,552 of Dann, Illingsworth and Gates, filed April 29, 1954.

Silver halide dispersions with our invention by forming an aqueous solution of the polymer and under agitated conditions either incorporating silver nitrate or alkali metal halide into that solution and running in a solution of the other or by .running solutions of both of these salts into the polymer solution. The solution may be in the pH range of 1.5 to 3.0 or it may be at a pH of 6 or more, such as may be obtained by the presence of ammonium hydroxide therein. After the silver halide dispersion is formed the soluble salts resulting from the reaction are removed by coagulating the dispersion such as by adjusting the pH to a range of 4-5 whereupon silver halide grains coagulate therefrom. These grains are separated from the liquid such as by settling and decanting. If desired the solid material may be washed with cold distilled water. If desired the silver halide may be prepared by using a peptizer such as gelatin and subsequently adding the polymer thereto so as to incorporate at least 2 parts thereof per part of solid material present with the pH outside of the coagulating range. Where the silver halide dispersion is prepared at a pH above 5 the acid subsequently employed by reducing the pH to the coagulation point may be any acid which will give the desired pH with any adverse effect on the photographic material. Dilute aqueous sulfuric acid, acetic acid or phosphoric acid are useful for this purpose.

The following examples illustrate our invention:

Example 1 750 ml. of distilled water were heated to 40 C. and there was dissolved therein 12 grams of a graft polymer as prepared by example 12 of Dann, Illingsworth and Gates application Ser. No. 426,552. 70 grams of potassium bromide and 0.7 gram of potassium iodide. The pH of the solution was adjusted to 6.0 and the temperature was elevated to 60 C. A solution of 85 grams of silver nitrate in 850 ml. of distilled water at 62 C. was introduced therein with mechanical agitation over a period of 20 minutes and the pH was adjusted to 4.0 by carefully adding 2.5 normal sulfuric acid. A coagulum immediately formed. After settling for 5 minutes the mother liquor was poured off and the coagulum was redispersed in 1500 ml. of distilled water at 40 C. and at a pH of 6. The mass was stirred for minutes and then recoagulated at pH 3.7. The coagulum was allowed to settle over a period of 5 minutes whereupon the liquid portion was removed and the coagulum was dispersed in 800 ml. of distilled water at a temperature of 40 C. and a pH of 6.0. 125 grams of photographic gelatin which had been soaked in 500 ml. of water was dissolved in the dispersion. normal sodium hydroxide and 3 cc. of 0.1 normal potassium bromide and 4.5 ml. of allyl thiourea in solution in methyl alcohol were added thereto. After heat ripening, the emulsion was coated on a cellulose acetate base,

are prepared in accordance The pH was adjusted to 7.5 with 2 dried, exposed for second on an Eastman Ib Sensitometer and developed for 4 minutes in Kodak Developer, D-19. A product having good photographic properties was obtained.

Example 2 The preceding example was repeated except that the polymer prepared in accordance with Example 7 of U. S. application Serial No. 426,551 of Illingsworth, Dann and Gates was employed as the peptizing material. A product having goodphotographic properties was obtained.

- Example 3 An experiment was carried out in the same manner as described in example 1 except that the polymer employed was that prepared in accordance with Example 15 of Ser. No. 426,552. A photographic film of good properties was found to have resulted,

. ,Example 4 A silver halide dispersion was prepared by mixing silver nitrate and potassium bromide in an aqueous solution of gelatin and after the formation of the silver halide, 24 parts of graft polymer as used in Example 1 was employed. The silver halide material thus obtained to which the polymer had been added Was coagulated by adjusting the pH to 6.0 whereupon coagulation occurred and the emulsion was washed. Upon redissolving and incorporating in an emulsion a product having good photographic properties resulted.

The following physical properties constitute a comparison of the emulsions obtained in accordance with the above examples, the results having been obtained by a ,5 second exposure in a 1B sensitometer and developing for 4 minutes in D-l9.

1. A method of preparing a washed silver halide dispersion which comprises precipitating silver halide in an aqueous solution of a gelatin-acrylic acid graft polymer, adjusting the pH of the mass to 4-5 whereby coagulation occurs, separating the coagulum from the liquid and redispersing in an aqueous liquid whereby a suspension substantially free of salts is formed.

2. A method of preparing a washed silver halide dispersion which comprises precipitating silver halide in an aqueous solution of a gelatin-acrylic acid-acrylamide graft polymer, adjusting the pH of the mass to 4-5 whereby coagulation occurs and redispersing the resulting coagulum in an aqueous liquid whereby a suspension substantially free of salt is formed.

3. A method of preparing silver halide dispersions substantially free of salts, which comprises preparing a composition of a peptized silver halide in an aqueous solution of a water-soluble graft polymer prepared by the emulsion copolymerization of 5-95% of a saturated protein material and 95-5% of a monomeric material, -100% of which is hydrophilic material selected from the group consisting of acrylic acid, methacrylic acid, acrylamides, vinyl pyridine and mixtures thereof and 40-0% of which is hydrophobic material, adjusting the pH of the mass to 4-5 whereby coagulation is caused, separating the liquid from the solid material formed and redispersing that solid material in an aqueous liquid.

4. A method of preparing silver halide dispersions substantially free of salts which comprises preparing a composition of a peptized silver halide in an aqueous solution of a water-soluble graft polymer prepared by the emulsion copolymerization of 595% of a saturated selected from the group consisting of acrylic acid, methacrylic acid, acrylamides, vinyl pyridine and mixtures thereof, adjusting the pH of the mass to 4-5 whereby coagulation is caused, separating the liquid from the solid material formed and redispersing that solid material in an aqueous liquid.

5. A method'of preparing silver halide dispersions substantially free of salts which comprises precipitating silver halide in an aqueous solution of gelatin, adding to the mass a water soluble graft polymer in the proportion of at least 2 parts of the polymer to 1 of solids in the mass, the graft polymer being that which results from the emulsion polymerization of a composition comprising 595% of a saturated protein material and 5% of a monomeric material, 60-100% of which is hydrophilic material selected from the group consisting References Cited in the file of this patent UNITED STATES PATENTS Damschroder et al Apr. 10, 1951 Yutzy et. all Oct. 21, 1952 

1. A METHOD OF PREPARING A WASHED SILVER HALIDE DISPERSION WHICH COMPRISES PRECIPITATING SILVER HALIDE IN AN AQUEOUS SOLUTION OF A GELATIN-ACRYLIC ACID GRAFT POLYMER, ADJUSTING THE PH OF THE MASS TO 4-5 WHEREBY COAGULATION OCCURS, SEPARATING THE COAGULUM FROM THE LIQUID AND REDISPERSING IN AN AQUEOUS LIQUID WHEREBY A SUSPENSION SUBSTANTIALLY FREE OF SALTS IS FORMED. 